1. Field of the Invention
This invention relates to the field of compensation for the inherent temperature dependence in automatic gain control amplifiers while retaining the exponential gain characteristic.
2. Background Art
The gain versus input control voltage for a typical automatic gain control (AGC) amplifier is usually an exponential function which results in constant control gain that is independent of the input signal amplitude. The constant gain characteristic allows the AGC feedback loop frequency compensation to be optimized for minimum transient settling time.
Previous technology for AGC amplifiers usually employ an emitter-coupled pair with diodes in a configuration such as a Gilbert Multiplier. However, the performance of these circuits suffer, since the exponential function is temperature dependent. The basic equation for the prior art AGC amplifier with an exponential gain control voltage is as follows: EQU V.sub.O -V.sub.O *=(V.sub.I -V.sub.I *)(R.sub.L /R.sub.E)exp[(q/kT)(V.sub.C *-V.sub.C)] EQU (V.sub.O -V.sub.O *)=differential output voltage EQU (V.sub.I -V.sub.I *)=differential input voltage EQU (V.sub.C -V.sub.C *)=differential control voltage EQU R.sub.L =load resistance EQU R.sub.E =emitter resistance EQU k=Boltzman's Constant EQU T=abolute temperature (degrees Kelvin) EQU q=electron charge
In this equation, the inherent dependency (q/kT) of the gain on temperature can be seen. When the gain (V.sub.O -V.sub.O *) versus (V.sub.C -V.sub.C *) of 25 dB range AGC is plotted, the gain range changes due to temperature variation by 8.4 dB over a 0.degree. C. to 127.degree. C. change.
To nullify this variation of gain, temperature compensated log-antilog exponential function amplifiers have been built using thermistor networks. The thermistor is a temperature-sensitive resistor that must be selected or trimmed for appropriate temperature compensation and be external to an integrated circuit.
Another prior art means of temperature compensation is accomplished by making the differential control voltage (V.sub.C -V.sub.C *) proportional to kT/q. In practice, however, this circuit is sensitive to the offset voltages in bipolar junction transistors. The circuit is also sensitive to the matching of the load resistors R.sub.L since their voltage drop distance appears as a control voltage difference. Furthermore, the circuit also has the disadvantage that a PTAT (proportional to absolute temperature) current source is required.
Therefore, it is an object of the present invention to compensate for the temperature dependency in the exponential gain control for an AGC amplifier.
It is another object of the present invention to compensate for the temperature dependence without using external devices.
It is another object of the present invention to suppress the temperature dependency without the disadvantage of requiring a PTAT device.